Our recent findings reveal that members of the neurotrophin gene family, in addition to supporting the survival and differentiation of multiple neuronal populations, also modulate synaptic transmission in the hippocampus. These novel trophin effects may play an important role in developmental plasticity and learning and memory. We now propose to identify the molecular basis of trophic regulation of synaptic plasticity, by integrating electrophysiological and molecular biochemical studies of synaptic membranes, isolated neurons, and brain slices. We have previously shown that application of BDNF or NT-4 enhances the strength of glutamatergic synapses via phosphorylation-dependent postsynaptic mechanism(s). Our initial electrophysiological experiments will combine whole-cell path clamp recording with iontophoretic application of transmitters to assess potential changes in receptor sensitivity. Specific protein kinase agonists and antagonist will be used to identify the signaling pathway(s) that mediate the trophin effect. Single-channel recordings will also be made to determine whether BDNF increase the number of active receptors or the properties of existing receptors. In parallel, we will perform biochemical studies of isolated postsynaptic densities (PSDs) to elucidate the specific molecular components activated by BDNF. We have recently shown that trkB, the high affinity receptor for BDNF, is an active component of the PSD, and activation of trkB increases phosphorylation of specific glutamate receptor subunits. The proposed studies will identify the trkB-associated primary effectors in the PSD that mediate this effect. Moreover, we will incorporate peptide mapping to identify the phosphorylated amino acid residues of target receptors. The latter stages of the project will focus on functional aspects, including trophic modulation of activity-dependent synaptic plasticity in hippocampal slices and contributions of local glial cells. Overall, we hope to contribute to our understanding of the multiple critical roles played by neurotrophins in the developing and adult brain.